Note that a dog would need to eat a surprisingly large amount of onions, garlic, or chocolate in order to get sick. According to the AKC:

Onion powder is in a surprisingly wide range of foods, from soups to baby food. It only takes 100 grams of onion (about the size of a medium onion) per 20 kilograms of a dog’s weight to cause toxic effects, which means that a 45-pound dog would only have to eat one medium-to-large onion to experience dangerous toxicity levels. Since most dogs would happily devour a bag of unattended onion rings or an onion casserole given the opportunity, this is a serious concern.

Scientific studies have found it takes approximately 15 to 30 grams of garlic per kilogram of body weight to produce harmful changes in a dog’s blood. To put that into perspective, the average clove of supermarket garlic weighs between 3 and 7 grams, so your dog would have to eat a lot to get really sick. However, some dogs are more sensitive to garlic toxicity than others, and consumption of a toxic dose spread out over a few days could also cause problems.

In simpler terms, that means a very concerning dose of chocolate is approximately one ounce of milk chocolate per pound of body weight. Since an average milk chocolate bar may be around 1.55 ounces, consuming even one chocolate bar can have serious consequences, especially for small dogs. Eating a crumb of chocolate cake or a very small piece of a chocolate bar, on the other hand, probably won’t kill your dog, especially if it is a larger breed.

One time my dog ate some chocolate and I was worried until I calculated that a dog his size (and he wasn’t a big dog) would have to eat an entire full-sized bar of dark chocolate before experiencing any symptoms at all. It’s probably best not to give dogs food containing small amounts of onions, garlic, or chocolate just in case, but there’s no need to worry if a dog eats human food containing small amounts of these ingredients.